1. Field of the Invention
The present invention relates to a CuInSe.sub.2 based thin film solar cells using a thin film CuInSe.sub.2 based compound semiconductor active layer, and a manufacturing method thereof.
2. Description of the Prior Art
CuInSe.sub.2 based thin film solar cells are expected to be used in inexpensive and large-sized solar cells similar to amorphous silicon (a-Si) thin film solar cells. Since CuInSe.sub.2 has a narrow optical band gap Eg of about 1.0 eV, it can photoelectrically convert light having long wavelengths. Such photoelectric conversion cannot be achieved in an a-Si solar cell film because a-Si has an optical band gap Eg of about 1.7 eV. Accordingly, it is desirable to provide an a-Si solar cell and a CuInSe thin film solar cell laminated together in order to form a high efficiency tandem thin film solar cell which can photoelectrically convert light of both short and long wavelengths.
FIG. 2 shows the structural components of a CuInSe.sub.2 thin film solar cell related to the present invention. The CuInSe.sub.2 thin film solar cell includes a readily available glass plate 1 having a smooth surface. Mo thin film 2 is deposited on a smooth upper-surface of glass plate 1 and serves as a back-surface electrode. A P-type CuInSe.sub.2 film 3 having a thickness of approximately 1.about.4 .mu.m is formed on Mo thin film 2. A transparent conductive n-type CdS or CdZnS film 4 having a thickness of about 0.05.about.0.1 .mu.mm is formed on P-type CuInSe.sub.2 film 3. A photovoltage generated when light 9 impinges upon this laminated structure is applied to a load 8 through terminals 6 and 7. Terminals 6 and 7 are provided on back-surface electrode 2 and on a ZnO film 5, respectively.
In manufacturing the above described CuInSe.sub.2 solar cell, the most important step involves forming the CuInSe.sub.2 film 3 active layer. Various methods have been proposed for making the CuInSe.sub.2 active layer, such as a three-source simultaneous-deposition method, a spraying method, a two-stage selenidation method, a selenidation method using H.sub.2 Se, a sputtering method, and an electrodeposition method. Of these methods, the three-source simultaneous-evaporation method, described by Nakata et al. in the magazine "Material Science", Vol. 25, p. 168 (1988), yields CuInSe.sub.2 solar cells having good characteristics. The three-source simultaneous-evaporation method requires a deposition apparatus having a vacuum tank which includes Cu, In, and Se evaporation sources. Cu, In, and Se are evaporated simultaneously from their respective evaporation sources and deposited on a substrate heated to 350.degree..about.400.degree. C. In this method, as well as in the above mentioned methods, it is important to control the composition of Cu, In, and Se in the CuInSe.sub.2 film to be formed.
FIG. 3 shows a process of manufacturing a conventional CuInSe.sub.2 thin film solar cell. In the last step of this process, after the CdS film is formed on the CuInSe.sub.2 layer, heat treatment is performed at about 200.degree. C. in O.sub.2 gas or air.
The present inventor fabricated CuInSe.sub.2 thin film solar cells having the structure shown in FIG. 2 using the three-source simultaneous-evaporation method incorporating the steps listed in FIG. 3. However, these CuInSe.sub.2 thin film solar cells were defective because the quality and characteristics of the CuInSe.sub.2 films employed varied depending upon the film-formation lots. Some of the CuInSe.sub.2 films had good characteristics while others had poor characteristics, even though the compositions of the CuInSe.sub.2 films were substantially the same. The composition distribution in a direction perpendicular to the plane of deposition of the CuInSe.sub.2 film (i.e. in a direction parallel to the thickness of the CuInSe.sub.2 film) was measured by an ion micro-analyzer, the composition distribution in the film surface was measured by an electron probe micro-analyzer, and the shape of the film surface was observed by a scanning-type electron microscope. The CuInSe.sub.2 crystal structure was inspected through X-ray diffraction or laser Raman spectroscopy, and quantitative analysis of the composition of the film was performed through ICP plasma light-emission analysis. None of these analytical methods, however, could differentiate between films having good and bad characteristics. Details of the process used to make the CuInSe.sub.2 thin film solar cell of FIG. 2 will be described below.
Back-surface electrode 2, having a thickness of 1.0 .mu.mm, was formed on glass substrate 1 by sputtering. P-type CuInSe.sub.2 thin film 3 was then deposited on back-surface electrode 2. P-type CuInSe.sub.2 thin film 3 has a two-layer structure slightly varying in composition. The overall thickness of p-type CuInSe.sub.2 thin film 3 is approximately 2.about.4 .mu.m. The substrate temperature during deposition of the first layer of p-type CuInSe.sub.2 film was 350.degree. C. and the Cu/In ratio was 1.1. The substrate temperature during deposition of the second layer of the CuInSe2 film was 450.degree. C. and the Cu/In ratio was 0.7. The average composition of the resulting two-layer CuInSe.sub.2 film was chemically analyzed through the ICP plasma light-emission analysis. As a result, in many CuInSe.sub.2 lots, Cu/In and Se/(Cu+In) were substantially constant so that Cu/in=0.85.about.1.0 and Se/(Cu+In)=1.0.about.1.1. The n-type CdS film 4 was formed by electron beam deposition to a thickness of about 0.1 .mu.m. ZnO film 5 was formed by sputtering of a ZnO target containing 2%-3% Al.sub.2 O.sub.3 and deposited to a thickness of about 1 .mu.m. Heat treatment was performed in dry air at 230.degree. C. for 2.about.10 hours.
The characteristic, that is, the conversion efficiency of the CuInSe.sub.2 thin film solar cells manufactured in the manner described above, showed values which varied significantly depending on the formation lots of the CuInSe.sub.2 films. Even in lots having substantially constant compositions of Cu/In=0.85.about.1.0 and Se/(Cu+In)=1.0.about.1.1, as determined by the chemical analysis of the CuInSe.sub.2, some of the solar cells had good conversion efficiency (.eta.) values of about 10% while others had extremely poor values of &lt;0.1%. Table 1 shows the composition of CuInSe.sub.2 films and values of cell characteristics (good and bad conversion efficiencies .eta.) for each of six lots in which the ratios of the constituent elements Cu/In and Se/(Cu+In) were varied within the ranges of 8.5.about.1.0 and 1.0.about.1.1, respectively.
TABLE 1 ______________________________________ CuInSe.sub.2 Film CuInSe.sub.2 Film Composition Solar Cell Forming (Analyzed Value) Characteristic Lot No. Cu/In Sn/(Cu + In) good/bad conv. eff. .sub..eta. ______________________________________ IC-112 0.978 1.03 good 9.3% IC-114 0.877 1.038 bad &lt;0.1% IC-115 0.927 1.02 bad &lt;0.1% IC-121 0.956 1.015 good 11.0% IC-125 0.959 1.033 bad &lt;0.1% IC-138 0.911 1.054 good 10.0% ______________________________________
For each of the six lots of the CuInSe.sub.2 films, in addition to the chemical analysis of the composition, vertical elemental-distribution analysis using an .ion micro-analyzer, element in-surface distribution analysis using an electron probe micro-analyzer, surface-shape observation using a scanning-type electron microscope, and crystal-structure observational studies through X-ray diffraction and laser Raman spectroscopy were also performed. However, none of the analytical methods could predict whether the characteristics of the CuInSe.sub.2 solar cell (i.e., conversion efficiency) would be good or bad.